Genomic instability and aneuploidy are hallmarks of human cancer and frequently correlate with poor prognosis. Despite this common observation, the mechanisms by which aneuploidy arises and its role in tumorigenesis remain subjects of intense interest and debate. Over the past several years, significant progress has been made in identifying the regulatory factors in mitosis. Some studies have shown an association between alterations in key mitotic factors and cancer. Specifically, prompted by the identification of Bub1 mutations in certain colon cancer cell lines, in vitro studies have shown that interference with this mitotic checkpoint protein can lead to aberrant mitosis and cooperation with Brca2 mutation in transformation. Another mitotic regulator, Aurora-A (Aur-A), is amplified or overexpressed in many human cancers , and specific Aur-A polymorphisms are associated with increased cancer susceptibility in humans and mice. However, direct proof of a role for Bub1 or Aur-A in tumorigenesis has not been demonstrated. Indeed, much remains to be learned about their roles in normal cells and during development. During the previous award, we developed several genetically engineered mouse (GEM) models to explore the roles of Bub1 and Aur-A in cell cycle regulation, development, and tumorigenesis. Included are transgenic lines expressing Aur-A or a dominant interfering Bub1 mutant, and several targeted endogenous mutants, including Cre-conditional deletion mutants of Aur-A and Bub1, a hypomorphic Aur-A, and gene-trap (likely null) Bub-1 mutant. During the next funding period our goal is to utilize these mutants to characterize the functions of Bub1 and Aur-A in cell cycle regulation, development and cancer. Although the NCI expected to fund the grant, the anniversary date for renewal was December of 2005, thus falling in the current budget year with the NCI payline at the 11th percentile. The grant is currently in a no- cost extension. The revised application includes substantial progress in characterizing all mutant mouse strains and has been reduced in scope and increased in detail as recommended by the reviewers. Our specific aims are to: (1) Analyze the cellular roles of Bub1 and Aur-A in mouse embryonic fibroblasts (MEFs), (2) Determine the phenotypic consequences of mutant Bub1 and Aur-A alleles in vivo, and (3) Determine whether impaired Bub1 and/or increased Aur-A activity can impact cancer development.